1. Field of the Invention
The present invention relates generally to telecommunication line circuitry and more specifically to circuitry for reducing the power dissipation in telephone subscriber line circuits with consequent reduction in the power requirements of the telephone central office. The present invention also relates generally to an electronic implementation of an ac transmission termination impedance.
2. Description of the Prior Art
The prior art of digital subscriber line/trunk circuitry having programmable signal generation and digitally implemented hybrid circuitry is illustrated by U.S. Pat. No. 4,161,633 of R. Treiber, assigned to the same assignee as is the present invention.
The prior art includes numerous techniques descriptive of circuitry which is designed to provide a constant line current for telephone subscriber lines. Examples of such circuitry is provided by reference to U.S. Pat. Nos. 3,035,122; 3,916,110; 4,056,691; 4,007,335 and 4,106,084. Essentially, the various circuits described in the referenced patents provide a constant or efficient line current for a telephone subscriber loop of variable length. U.S. Pat. No. 4,161,633 of Robert Treiber assigned to the same assignee as is the present invention describes a telephone line circuit providing an interface between telephone subscriber lines and a digital circuit wherein a programmable signal generator circuit senses subscriber line transmission conditions; and in response to such first subscriber line transmission conditions, derives control signals which advance the effective line matching impedance in accordance with the first subscriber line conditions. A technique for reducing power dissipation and varying the subscriber line feed circuit characteristics without varying the feed resistance value is described in copending U.S. patent application Ser. No. 098,104 filed Nov. 28, 1979 by Ramon Chea entitled Apparatus for Regulating Current Supplied To A Telephone Line; now U.S. Pat. No. 4,313,106 and is assigned to the same assignee as is the present application. While said copending application discloses a technique for regulating dc line feed current to a telephone subscriber loop, wherein power dissipation in the line feed resistors associated with the telephone subscriber interface circuit is reduced, while required loop current versus loop resistance characteristics is provided.